1. Field of the Invention
The present invention relates to the design and fabrication of gallium nitride (GaN) semiconductor light-emitting devices. More specifically, the present invention relates to methods for fabricating GaN-based semiconductor light-emitting devices with ultra-high reverse breakdown voltages.
2. Related Art
Recent success in the development of gallium nitride-based light emitting devices (LEDs) and lasers not only extends the light-emission spectrum to the green, blue, and ultraviolet region, but also can achieve high light emission efficiency. This is because GaN materials have a large direct-band-gap to allow photonic emission in these spectra. Because of their high energy-efficiency, high brightness, and long lifetime, GaN-based semiconductor LEDs have been widely used in applications including full-color large-screen displays, traffic lights, backlight sources, and solid-state lighting.
A GaN-based LED typically includes a p-n junction structure. A GaN-LED can also include a GaN-based n-type semiconductor layer, a multiple-quantum-well (MQW) active region, and a GaN-based p-type semiconductor layer. During light emission, the p-n junction or MQW region is forward-biased at a voltage which causes a current to flow from the p-type layer to the n-type layer through the active layer. However, the LED can sometimes become reversely biased due to a number of reasons, for example, being subject to electrostatic discharge (ESD) or being accidentally connected to a reverse voltage source. Furthermore, an LED becomes reversely biased when an alternating voltage is used to drive the LED.
When the LED is reversely biased, little current (referred to as a “reverse current”) flows through the p-n junction until the reverse voltage reaches the level of the reverse breakdown voltage of the GaN-based LED. When the bias voltage exceeds the breakdown voltage, even momentarily, the reverse current increases significantly which could lead to permanent damage to the LED. Note that a reverse bias due to ESD can be particularly harmful because the ESD voltage can be much higher than the typical reverse breakdown voltage of an LED, and the occurrence of an ESD event is often unpredictable. Conventional GaN-based LEDs typically exhibit low reverse breakdown voltages, and hence are vulnerable to these reverse breakdown risks.
It is generally desirable to fabricate GaN-based LEDs with high reverse breakdown voltage to increase the reliability of the LED. However, it has been difficult to achieve high reverse breakdown voltages in practice.